Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semiconductive, and insulating materials that are patterned and etched to form integrated circuits (IC's).
For many years in the semiconductor industry, optical lithography techniques such as contact printing, proximity printing, and projection printing have been used to pattern material layers of integrated circuits. Projection printing is commonly used in the semiconductor industry, using wavelengths of 248 nm or 193 nm, as examples. At such wavelengths, lens projection systems and transmission lithography masks are used for patterning, wherein light is passed through lithography masks to impinge upon a wafer.
However, as the minimum feature sizes of IC's are decreased, the semiconductor industry is trending towards the use of very short wavelength, immersion lithography technologies or non-optical lithographic techniques to achieve the decreased feature sizes demanded by the industry.
For lithographic printing of integrated circuit patterns below about 50 nm feature sizes, extreme ultraviolet lithography (EUVL) technology using light in the soft x-ray range (e.g., about 10 to 15 nm) is under development. Ultraviolet (UV) light has a shorter wavelength than visible light. For example, UV light is usually considered to fall within the wavelength range of about 157 to 400 nm. In EUVL, extreme UV (EUV) light, having a shorter wavelength than UV light, e.g., about 13.5 nm, is used as the wavelength. In EUVL, plasma is used to generate a broadband radiation with significant EUV radiation. The plasma is either generated by laser radiation bombarding a target material such as Xe or Sn or by an electrical discharge, as examples. The EUV radiation is collected by a system of mirrors coated with EUV reflecting interference films, also known as Bragg reflectors. The EUV radiation is then used to illuminate an EUV reflection lithography mask. The pattern on the lithography mask is imaged and de-magnified onto a resist-coated wafer. The entire lithography mask pattern is exposed onto the wafer by synchronously scanning the lithography mask and the wafer.
EUVL is a reflective lithographic technology using mirror elements coated with EUV Bragg reflectors, also referred to in the art as EUV multi-layers. EUV radiation is strongly absorbed in most materials, even gases; therefore, EUV imaging must take place in a very well controlled vacuum environment that reduces rest gas absorption of EUV radiation and protects the multi-layer mirror element surfaces from contaminants. In some types of high vacuum systems, the system can be subjected to a high temperature “bake” or heating process, to lower the partial pressure of water vapor in the system. However, EUVL systems cannot be baked or subjected to extremely high temperatures, because the optics alignment and quality of the multi-layer mirror elements in the EUVL system would be damaged, due to possible intermixing of the Mo/Si superlattice that the EUV optics are comprised of. Thus, the residual water vapor partial pressure that is present (because the system cannot be baked) in EUVL systems causes significant oxidation of the surfaces of the multi-layer mirror elements, if no preventive measures are taken. Oxidation of the multi-layer mirror elements reduces the lifetime of the multi-layer mirror elements and also results in reduced reflectivity, degrading the performance of the EUVL system.
One of the preventive measures to avoid oxidation of the multi-layer mirror element surfaces in EUVL systems that is being explored is the development of protective capping layers. The protective capping layers are deposited on top of the multi-layer mirror element surfaces to protect them from oxidation. However, the capping methods and layers under development have been found to be inadequate in preventing oxidation of the multi-layer mirror element surface. Prior art capping layers temporarily, e.g., for about 230 hours, prevent oxidation, which is insufficient oxidation prevention for production tools that need to remain oxide free for about 30,000 hours, for example. Oxidation of the multi-layer mirror elements results in reduced reflectivity of the multi-layer mirror element, and causes early life failures of the multi-layer mirror element.
What are needed in the art are methods of increasing optics lifetime in EUV lithography systems.